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Open star cluster, artwork. An open star cluster is a loose grouping of stars within a galaxy. The stars tend to be young, sometimes forming from the same nebula, but can be of any age. The number of stars can be in the hundreds, but can also be much less. Over time, due to the gravitational forces exerted by other stars in the galaxy, the stars of an open star cluster will drift apart. The closest open star cluster to Earth is the Hyades open star cluster, which is 150 light years away and in the constellation of Taurus.